In situ combustion process



ilnited States Patent 3,019,837 IN SITU COMBUSTION PROCESS John W. Marx, Richard H. Langenheim, Henry 0. Dixon, 7

and Joseph C. Trautham, all of Bartlesville, Okla, assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Oct. 28, 1957, Ser. No. 692,592 8 Claims. (Cl. 16611) naceous strata is coming into wider use in the petroleum industry. In this production technique, an area of the formation is heated to combustion supporting temperature by any suitable method and combustion is initiated by contacting the hot stratum with an oxygen-containing gas such as air or oxygen-enriched air. Because of the high heat capacity of the stratum and the heat transfer characteristics thereof extended periods of heating, such as several days and even weeks, are usually required to bring a section of the formation up to the ignition temperature of the carbonaceous material therein. The ignition temperature can be lowered by utilizing pure oxygen or oxygen-enriched air, but this of course increases the cost of recovery.

The principal object of the invention is to provide an improved process for effecting oil recovery from a carbonaceous stratum by in situ combustion. Another object is to provide an improved process for initiating in situ combustion in a carbonaceous stratum. A further object is to reduce the time required for initiating in situ combustion in a carbonaceous stratum. It is also an object of the invention to increase the O availability in carbonaceous stratum for initiation of in situ combustion therein and for moving the combustion zone thru the stratum. Other objects of the invention will become apparent when considering the accompanying disclosure.

It has been found that natural, porous tar sands, oil sands, and shales made permeable by fracturing methods, or naturally permeable, absorb 0 when air or other free oxygen-containing gas is injected into such strata. This phenomenon has been observed when air injected into a stratum has been found to be deficient in oxygen when the air is withdrawn or released from the formation. Laboratory tests confirm this observation that oxygen is selectively absorbed from the air by native tar sands. When in situ burning of carbonaceous material in a stratum is effected after injecting air into the stratum prior to the initiation of combustion, as much as 3% CO in excess of that theoretically possible from the oxygen content of the air has been found in the combustion gas recovered, the additional 0 coming from absorbed oxygen present in the stratum before the combustion takes place.

The process of the invention comprises contacting a carbonaceous stratum with free oxygen, such as with air, so as to effect substantial absorption of 0 by the stratum, thereafter heating the stratum in the area of the absorbed oxygen to a combustion supporting temperature and, while the stratum is at such temperature, contacting the same with a suitable free-oxygen containing gas so as to initiate combustion of the carbonaceous material with the aid of the absorbed oxygen. In order to effect the contacting of the stratum with free-oxygen an ignition borehole is drilled thru the stratum and one or more surrounding boreholes are also drilled therein so that air can be passed laterally thru the stratum, either from the ignition borehole or from the surrounding boreholes.

In formations which are impermeable or slightly permeable, fracturing can be utilized to provide suitable channels for passage of air thru the stratum to effect the desired absorption of O The flow of air thru the formation can be continuous or intermittent as the oxygen is rather quickly absorbed by the carbonaceous material. When the section of the formation adjacent the ignition borehole is substantially saturated with 0 this section of the formation is heated by any suitable means, such as by burning a mixture of fuel gas and oxygen adjacent the wall of the borehole containing the carbonaceous material or by an electric heater or other suitable means, and when the temperature of the formation adjacent the borehole reaches a minimum of about 450 F. and preferably 500 to 600 F., injection of air into the formation is commenced and ignition of the carbonaceous material is effected. A preferred method of initiating the combustion comprises injecting air into the surrounding boreholes so that the air passes thru the stratum to the ignition borehole, causing the ignition of the hot carbonaceous material surrounding the ignition borehole, and effecting movement of the resulting combustion zone radially outwardly from the ignition borehole countercurrently to the flow of air so that the combustion zone passes thru the stratum to the injection boreholes.

If the injection of air is continued after the combustion zone arrives at an injection borehole the movement of the combustion zone is reversed and it passes back thru the formation, feeding on the carbonized residue therein eflfected in the first burning stage, until the combustion zone finally arrives at the ignition borehole. In strata which are highly permeable, it is feasible to initiate combustion in the ignition borehole by direct injection of air and continuing the injection of air thru this borehole so as to movethe combustion zone by direct drive to the surrounding boreholes. It is also feasible to initiatecombustion by direct air injection thru the ignition borehole and after a substantial section of the stratum around the borehole has been heated up to the range of 1,000 to l,500 F. or higher in temperature, the flow of direct air can be cut off and inverse air, injected thru the surrounding boreholes, can be utilized to continue the combustion and move the combustion zone thru the formation to the surrounding boreholes.

The absorption process is non-exothermic or only mildly exothermic, indicating that oxygen is merely held in solution in the hydrocarbon, Field data indicate that, if sufficient residence time is allowed, more than percent of the oxygen can be removed from air by passing air thru a permeable carbonaceous stratum. This oxygen is then available to assist, and possibly to completely sustain active combustion of the carbonaceous material when temperatures are subsequently raised to the ignition point.

The invention is not limited to tar sands alone but is applicable to shales, Athabasca sands, oil sands, etc., if the stratum is permeable or has been made permeable by fracturing methods or by acid treatment or other suitable means. The method of the invention is more effective in strata containing non-liquid hydrocarbons than in strata containing a large proportion of the hydrocarbon material in liquid form.

To illustrate the invention, boreholes were drilled in a tar sand in a conventional 5-spot pattern, with the corner wells 10 feet apart and the ignition well at the center of the pattern. Air was injected thru the corner wells at a total rate of about 2,000 cubic feet per hour and passed thru the formation to the ignition well. The formation was about 10 feet in thickness. The first air recovered from the ignition borehole had an oxygen content in the range of 10 to 11 percent and after about 16 hours of flow the absorption of oxygen was substantially comamass? plate, as the oxygen content of the withdrawn air was normal. Laboratory tests indicate that this tar sand absorbs oxygen until its weight increases by about 3 percent. After saturation of the tar sand was complete, the flow of air was terminated, temporarily, and the wall of the ignition bore-hole was heated by a burner positioned in the borehole opposite the carbonaceous stratum by burning a mixture of propane and air therein. When the stratum surrounding the burner was brought to a combustion supporting temperature, the how of air from the corner wells was again initiated and the combustion of the carbonaceous material was immediately initiated so that the resulting combustion Zone moved slowly thru the stratum toward the injection wells. Hydrocarbons produced during the in situ combustion were recovered from the ignition well by conventional methods.

In the pattern described above, time for saturation of the stratum between the injection boreholes and the ignition borehole will vary with different strata as to the character of the carbonaceous materials and the permeability of the strata. This time usually ranges from about 10 to about 100 hours. The how of air can be either continuous or intermittent, since oxygen is absorbed from stagnant air but, after the oxygen content of the air is depleted or substantially reduced, more e'fiicient absorption is obtained by introducing fresh air. The invention is not limited to the spacing described in the above illustration but may be varied as to the number of injection wells as well as to the distance between the ignition well and the injection wells. It is to be understood that the saturation of the carbonaceous material in the stratum with oxygen not only aids the initiation of combustion but also enhances the combustion process as the combustion front moves thru the stratum.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. A process for initiating in situ combustion in a permeable stratum containing carbonaceous material which comprises contacting said formation adjacent an ignition borehole therein with a fr'ee-oxygen-containing gas so as to effect substantial absorption of by said stratum and withdrawing gaseous efiiuent from said stratum comprising contacting gas; continuing contacting of said stratum with said gas for a period of at least hours and until the concentration of O in the efliuent gas from the contacted stratum is substantially the same as in said gas initially; thereafter heating said stratum adjacent said borehole to a combustion supporting temperature; and while said stratum is at said temperature, contacting same with a freeoxygen-containing gas so as to initiate combustion of said carbonaceous material with the aid of the absorbed oxygen.

2. The process of claim 1 wherein air is passed thru said stratum to said borehole from at least one other borehole to effect said absorption and after said stratum is heated, air injected thru said other borehole is utilized for initiating said combustion so that the combustion zone advances counter-currently .to said air.

3. The process of claim 1 wherein air is injected into said stratum thru said ignition borehole to effect said absorption.

4. The process of claim 1 wherein said contacting is with continuously moving air.

5. The process of claim 1 wherein said contacting is with stagnant air during a substantial portion of the contacting time.

6. The process of claim 1 wherein air in intermittently injected into said stratum.

7. A process for initiating in situ combustion in a permeable stratum containing carbonaceous material which comprises injecting air into said stratum at a plurality of points around a'nignition' borehole and forcing said air thru said stratum to said borehole; continuing the contacting of saidstratum with air until at least a substantial section of said stratum around said ignition borehole is saturated with O thereafter, heating said stratum adjacent said borehole to combustion supporting temperature; while said stratum is at said temperature, contacting same with air so as to initiate combustion therein; and continuing injection of air at said points so as to move the resulting combustion zone thru said stratum toward said injection points.

8. A pieces for initiating in situ combustion in a permeable stratum containing combustible carbonaceous material which comprises passing air thru a section of said stratum between a first well and second well therein so as to effect absorption of 0 by said stratum; continuing the passing of air thru said stratum for at least 10 hours and until the 0 concentration in the effiuent gas is substantially that of the injected air; thereafter igniting said stratum around one of said wells to establish a combustion zone in the adjacent stratum; and moving said combustion zone thru said stratum between the wells by feeding air thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,793,696 Morse May 28, 1957 2,862,557 Van Utenhove et al Dec. 2, 1958 2,917,112 Trantham et al. Dec. 15, 1959 

8. A PROCESS FOR INITIATING IN SITU COMBUSTION IN A PERMEABLE STRATUM CONTAINING COMBUSTIBLE CARBONACEOUS MATERIAL WHICH COMPRISES PASSING AIR THRU A SECTION OF SAID STRATUM BETWEEN A FIRST WELL AND SECOND WELL THEREIN SO AS TO EFFECT ABSORPTION OF 02 BY SAID STRATUM; CONTINUING THE PASSING OF AIR THRU SAID STRATUM FOR AT LEAST 10 HOURS AND UNTIL THE O2 CONCENTRATION IN THE EFFLUENT GAS IS SUBSTANTIALLY THAT OF THE INJECTED AIR; THEREAFTER IGNITING SAID STRATUM AROUND ONE OF SAID WELLS TO ESTABLISH A COMBUSTION ZONE IN THE ADJACENT STRATUM; AND MOVING SAID COMBUSTION ZONE THRU SAID STRATUM BETWEEN THE WELLS BY FEEDING AIR THERETO. 